Device Not Available on PyUSB

Question

Playing around with PyUSB a bit to see if it offers some insight as to why a WebUSB library I'm using isn't finding my device. I installed libusb1 on the Mac via Homebrew with brew install libusb .

Ran lsusb -vv to get device details. Also set a couple of environment variables for PyUSB:

export PYUSB_LOG_FILENAME=pysubdebug.log and export PYUSB_DEBUG=debug

import usb

VENDOR_ID = 0x0483
PRODUCT_ID = 0x5740
DATA_SIZE = 1

device = usb.core.find(idVendor=VENDOR_ID, idProduct=PRODUCT_ID)

>>> device
<DEVICE ID 0483:5740 on Bus 020 Address 014>

>>> device.is_kernel_driver_active(0)
False

>>> device.set_configuration()
Traceback (abridged)
usb.core.USBError: [Errno 19] No such device (it may have been disconnected)

>>> cfg = device.get_active_configuration()
Traceback (abridged)
usb.core.USBError: [Errno None] Configuration not set

From the log file it looks like python (which is in a venv) is using a file located at /usr/local/lib/ :

2020-10-27 12:20:41,706 DEBUG:usb.backend.libusb1:_LibUSB.__init__(<CDLL '/usr/local/lib/libusb-1.0.dylib', handle 7f8ba652c7f0 at 0x1101a5f70>)
2020-10-27 12:20:41,712 INFO:usb.core:find(): using backend "usb.backend.libusb1"

Brewed files:

$brew ls libusb
/usr/local/Cellar/libusb/1.0.23/include/libusb-1.0/libusb.h
/usr/local/Cellar/libusb/1.0.23/lib/libusb-1.0.0.dylib
/usr/local/Cellar/libusb/1.0.23/lib/pkgconfig/libusb-1.0.pc
/usr/local/Cellar/libusb/1.0.23/lib/ (2 other files)
/usr/local/Cellar/libusb/1.0.23/share/libusb/ (13 files)

And confirming that PyUSB is looking in the right location:

ls -l /usr/local/lib/libusb-1.0.dylib
lrwxr-xr-x  #details# /usr/local/lib/libusb-1.0.dylib@ -> ../Cellar/libusb/1.0.23/lib/libusb-1.0.dylib

Do I need to be creating a function to Specify a library by hand ? Doesn't seem like that's the issue.

Maybe there's a configuration step I'm missing.

Answer 1

Being used to http:// data transfer, USB seems to be more fragile and platform specific.

For example, where I can generally load a web app in as many browsers as I like, a USB device can only be "claimed" by a single device, app (and perhaps process).

I found a (commercial) app, called Serial , through which I could (using the free demo) confirm that my device either is or isn't "claimed".

Some Apple users were able to use kextunload to "unclaim" a device.

On OSX, the kextstat terminal app yielded some details about which kernels were claiming which devices:

kextstat | grep usb
   22    8 0xffffff7f813ec000 0x8000     0x8000     com.apple.driver.usb.AppleUSBCommon (1.0) C2917767-E187-3F86-8E1D-3342A98EF53A <6 5 3 1>
   53    0 0xffffff7f817cf000 0x5000     0x5000     com.apple.driver.usb.AppleUSBHostPacketFilter (1.0) 2569DC26-1911-36D4-9BE3-A727E9535BB2 <23 22 8 7 6 5 3 1>
   54    1 0xffffff7f816a8000 0x56000    0x56000    com.apple.driver.usb.AppleUSBXHCI (1.2) 0E02208C-A8FC-3966-9C74-F09EF887E7E7 <23 22 12 8 7 6 5 3 1>
   etc...

But I kept getting an error that they were "in use" and couldn't be unloaded.

Ultimately I REBOOTED the computer, with the USB device plugged directly in using a good USB cable, and thanks to the PyUSB tutorial docs and this post was able to at least pull in some data from the device with this code:

import usb.core
import usb.util

# got these using the command lsusb -vv
VENDOR_ID = 0x0483
PRODUCT_ID = 0x5740
DATA_SIZE = 1

device = usb.core.find(idVendor=VENDOR_ID, idProduct=PRODUCT_ID)

device.is_kernel_driver_active(0)

device.set_configuration()

device.read(0x81, 255, 1000000)

Look:

array('B', [51, 48, 51, 32, 86, 49, 46, 48, 50, 32, 53, 56, 51, 51, 98, 49, 49, 56, 32, 48, 48, 48, 48, 48, 48, 48, 48, 32, 48, 48, 48, 48, 48, 48, 48, 48, 32, 48, 48, 48, 50, 57, 99, 97, 54, 32, 48, 48, 48, 48, 48, 48, 48, 54, 13, 10])

Gotcha

When I first tried to read the timeout was too short and/or the buffer was too small and then subsequent read requests would return usb.core.USBError: [Errno 32] Pipe error .

I imagine this is because python was busy trying to do something or needed to be disconnected. The workaround was just to exit() python and try again with higher parameters.